17. The chemical formulas for propane - C3H8 - and butane - C4H10 - are different. However, the formula for butane and isobutane are the same. Isobutane (i-butane) is an isomer of butane, so it has a different arrangement of its atoms.

18. Autogas is typically a mixture of propane and butane but can be all propane, too.